Method of producing product including silicon wires

ABSTRACT

A product including a plurality of wires, in which longitudinal directions of the wires are arranged in one direction so that each one of the wires is positioned end to end to one another, and a method of producing the same are disclosed. The longitudinal directions of the plurality of wires each covered with a polymer are arranged in one direction in a solvent, and the plurality of the wires whose longitudinal directions are arranged in one direction is fixed by using the polymer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of co-pending U.S. patentapplication Ser. No. 11/687,064 filed Mar. 16, 2007, to which priorityunder 35 U.S.C. §120 is claimed. The contents of application Ser. No.11/687,064 are hereby incorporated by reference for all purposes as iffully set forth herein. This application also claims foreign prioritybenefits under 35 U.S.C. §119 to Japanese Patent Application No.2006-101879 filed Apr. 3, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a product including silicon wires and amethod of producing the same.

2. Description of the Related Art

Silicon wires are wire-like molecules having diameters of severalnanometers to several tens nanometers, specifically, 2 nm to 30 nm, forexample, and lengths of 0.05 micrometers to several hundredsmicrometers, specifically, 0.1 μm to 300 μm, for example.

The silicon wires are sometimes referred to as “silicon nanowires”because they are of sizes of the nano-order.

A silicon wire is formed by a production method therefor involving, forexample, allowing Au nanoparticles to be monodispersed on a siliconwafer, and allowing Si to grow in SiH₄ gas through high-temperaturechemical vapor deposition (high-temperature CVD) by using as a catalystthe monodispersed Au nanoparticles (see, Charles Lieber, Appl. Phys.Lett., 78, 2214-2216 (2001)).

The silicon nanowires are characterized in that they are extremely thinand long, they have structural stability and excellent electricalproperty, and their electrical properties are easily controlled.

From the viewpoints of such characteristics, the silicon nanowires areexpected to be utilized as an electronic material.

For example, as shown in FIG. 4, high performance transistors can beproduced by conducting doping to a silicon nanowire 41 to form an n-p-njunction and providing a gate insulating film 44 and electrodes. In FIG.4, a reference numeral 42 denotes a source electrode, a referencenumeral 43 denotes a drain electrode, a reference numeral 45 denotes agate electrode, a reference numeral 46 denotes a TFT substrate, areference numeral 47 denotes a semiconductor layer, and a referencenumeral 48 denotes a protective layer.

The silicon nanowires are extremely thin. Therefore, as shown in FIG. 5,silicon nanowires 51 are sometimes arranged on electrodes at variousangles between devices (in FIG. 5, a source electrode 52 and a drainelectrode 53).

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a product including aplurality of silicon wires aligned in a particular direction, and amethod of producing the same.

According to a first aspect of the present invention, there is provideda method of producing a product including silicon wires, including:arranging longitudinal directions of a plurality of silicon wires eachcovered with a polymer in a solution in one direction; and fixing theplurality of the silicon wires whose longitudinal directions arearranged in one direction by using the polymer.

According to a second aspect of the present invention, there is provideda method of producing a product including silicon wires, including:covering a plurality of silicon wires with a hydrophobic polymer havingaffinity to the silicon wires in a solvent; applying at least one of analternating electric field and an alternating magnetic field to theplurality of silicon wires each covered with the hydrophobic polymer;and fixing a group of the plurality of silicon wires each covered withthe hydrophobic polymer by applying thereto one of an ultraviolet rayand an infrared ray or by applying heat.

In this case, before the group of the silicon wires is fixed, the groupof the silicon wires can be scooped up on a substrate from the solventand then be fixed.

According to a third aspect of the present invention, there is provideda method of producing a product including a plurality of wires,including: arranging longitudinal directions of a plurality of wireseach covered with a polymer in a solution in one direction; and fixingthe plurality of wires whose longitudinal directions are arranged in onedirection by using the polymer.

According to a fourth aspect of the present invention, there is provideda product including a plurality of silicon wires, in which longitudinaldirections of the silicon wires are arranged in one direction and thesilicon wires are fixed with one another via a polymer.

Alternatively, another aspect of the present invention includes thesteps of: stirring the silicon nanowires and the hydrophobic polymerhaving affinity to the silicon nanowires in a solvent to cover thesilicon nanowires with the hydrophobic polymer having affinity to thesilicon nanowires; applying one of an alternating electric field and analternating magnetic field to the silicon nanowires each covered withthe hydrophobic polymer having affinity to the silicon nanowires in asolvent having a polarity opposite to that of the silicon nanowires; andfixing the silicon nanowires each covered with the hydrophobic polymerhaving affinity to the silicon nanowires by applying one of anultraviolet ray and an infrared ray or by applying heat, wherein thesilicon nanowires each covered with the hydrophobic polymer havingaffinity to the silicon nanowires are aligned in a uniform directionthrough the step of applying one of an alternating electric field and analternating magnetic field.

As described above, according to the present invention, there areprovided a product including silicon wires aligned in a particulardirection, and a method of producing the same.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D and 1E show a method of producing a silicon wiresheet or a silicon wire ribbon of the present invention (Example 1).

FIGS. 2A, 2B, 2C, 2D and 2E show a method of forming a silicon wiresheet or a silicon wire ribbon of the present invention on a substrate(Example 2).

FIGS. 3A and 3B show the method of forming a silicon wire sheet or asilicon wire ribbon of the present invention on a substrate (Example 2).

FIG. 4 shows a structure of a TFT device using the silicon wires of thepresent invention.

FIG. 5 shows an arrangement of silicon wires of a conventional exampleand Comparative Example 1.

FIGS. 6A and 6B show a process of transferring the silicon wire sheet orthe silicon wire ribbon of the present invention onto a device.

DESCRIPTION OF THE EMBODIMENTS

Now, the product including silicon wires and a production methodtherefor of the present invention will be described by referring toFIGS. 1A, 1B, 1C, 1D and 1E.

In the present invention, the silicon wires are wire-like moleculeshaving diameters of several nanometers to several tens nanometers,specifically, 2 nm to 30 nm, for example, and lengths of 0.05micrometers to several hundreds micrometers, specifically, 0.1 μm to 300μm, for example. Since the silicon wires are of the nano-meter order,they are sometimes referred to as “silicon nanowires” hereinafter.

(Production Method)

Descriptions are given to the method of producing the product includingsilicon wires.

First, longitudinal directions of a plurality of silicon wires eachcovered with a polymer are arranged in one direction in a solution.

Then, the plurality of the silicon wires whose longitudinal directionsare arranged in one direction is fixed by using the polymer.

Thus, a group of silicon wires in which the plurality of the siliconwires are orientated in a particular direction is obtained.

The polymer to be used in the present invention is not particularlylimited as long as it can be used for the fixation in a later step.Covering with the polymer is performed, for example, as described below.

A silicon wire is covered with a hydrophobic polymer having affinity tothe silicon wire in a solvent. More specific descriptions are givenhereinlater.

For arranging the longitudinal directions of the plurality of thesilicon wires each covered with the polymer in one direction, at leastone of an alternating electric field and an alternating magnetic fieldis used, for example.

The group of the plurality of silicon wires each covered with thepolymer is irradiated with one of an ultraviolet ray and an infrared rayor heated to fix the group of the silicon wires.

Note that, at least one of an alternating electric field and analternating magnetic field can be applied to the silicon wires in asolvent having a polarity opposite to that of the silicon wires.

The application of one of the alternating electric field and thealternating magnetic field allows the group of the silicon wires toalign in one direction.

Any one of polymers having insulating property can be used as thepolymer.

Note that, the group of the silicon wires can be scooped up on asubstrate from a solvent before the group of the silicon wires is fixed.

Descriptions are given by taking the silicon wires as an example.However, the present invention is not particularly limited to wires ofsilicon. For example, carbon nanotubes, and any one of wires andwire-like products each including a compound semiconductor such as SiGeand GaAs can be used.

According to the present invention as described above, there can beprovided a product including a plurality of silicon wires, in whichlongitudinal directions of the plurality of the silicon wires arearranged in one direction and fixed with one another via a polymer.

Note that, each of the silicon wires sometimes has a natural oxide filmor an oxide film formed through oxidization of a surface layer thereofprovided on the surface thereof. The silicon wires to be used in thepresent invention of course encompass wires each having an oxide film ona surface thereof.

Hereinafter, the method of producing the product of the presentinvention is described in detail by referring to the drawings. However,the present invention is not limited to the following production method.

First, as shown in FIG. 1A, a silicon nanowire 11 and a hydrophobicpolymer 12 having affinity to the silicon nanowire are dispersed in asolvent.

Next, as shown in FIG. 1B, the silicon nanowire is each covered with thepolymer (FIG. 1BP) by stirring the solution a of FIG. 1A.

Next, the silicon nanowire covered with the polymer is dispersed in asolution of a solvent b having a polarity opposite to that of thesilicon nanowire. The solution is then applied with one of analternating electric field and an alternating magnetic field at afrequency of several MHz, whereby silicon nanowire polymers 13 arealigned in a monolayer on a surface of the solution as shown in FIG. 1C.

Next, the aligned silicon nanowire polymers 13 are irradiated with anyone of an ultraviolet ray 14 and the like to cause one of crosslinkingreaction and polymerization reaction, to thereby fix the aligned siliconnanowire polymers. The fixation may be performed by using an infraredray or by heating.

By the method as described above, there can be formed one of a siliconnanowire sheet 15 and a silicon nanowire ribbon 15 in which the siliconnanowires are aligned.

In this case, the reasons for covering the silicon nanowire 11 with thepolymer 12 are as follows: (1) the silicon nanowires do not overlap withone another; (2) insulating property can be ensured; (3) gooddispersibility in a solvent can be attained; (4) a monomoleculararrangement (LB film formation) is easily formed; (5) a structuredmaterial can be fixed by one of the crosslinking reaction and thepolymerization reaction of the polymers; and the like.

For the polymer 12, any kind of polymers can be used as long as theyhave affinity to the surface of the silicon nanowire 11. For example, itis preferable to use a polymer having one of a Si atom and an-conjugated bond in its molecular skeleton. In addition, examples of aninsulating polymer include polyethylene, polystyrene, and polyethyleneterephthalate. In addition, examples of polymers which strongly bind toone another by one of the crosslinking reaction and the polymerizationreaction preferably include polyarylene ethynylene, polydiacetylene,polyvinyl pyridine, and polydibenzodisilazepine.

For fixing the silicon nanowire polymers 13, one of the crosslinkingreaction and the polymerization reaction of the polymers is used. One ofirradiation with one of an ultraviolet ray and an infrared ray andapplication of heat is selected depending on the kind of the polymer.

For the solvent a 16 (solvent a 26 in FIG. 2A), any kind of solvents canbe used as long as they can dissolve the polymer. Examples of thesolvent include 1,2-dichloroethane and tetrahydrofuran.

For a procedure of forming the silicon nanowire sheet or ribbon 15 on asubstrate such as glass, a flexible substrate such as tape or a sheet,or the like, the Langmuir-Blodgett (LB) method is typically used. By theprocedure, as shown in FIG. 2D, polymers aligned in a monomolecular filmon an aqueous solution 27 are scooped up on a substrate 28 from thesurface of the aqueous solution, and the monomolecular arrangement ofthe polymers can be directly transferred onto the substrate. The siliconnanowires 21 can be aligned in such a manner that longitudinaldirections thereof are perpendicular to the direction of lifting-up ofthe substrate as shown in FIG. 3A. Alternatively, the silicon nanowirescan be aligned in such a manner that the longitudinal directions thereofare parallel to the direction of lifting-up of the substrate as shown inFIG. 3B. In the case of FIG. 3A, the longitudinal directions of thepolymers can be controlled by the direction of one of the electric fieldand magnetic field to be applied. In the case of FIG. 3B, siliconnanowire polymers 33 are naturally aligned approximately in a directionof a flow path, and are scooped up on a substrate 38 by moving thesubstrate in a direction parallel to the direction of the flow path.

As described above, in the case where the silicon nanowire polymers 23are formed on the substrate by one of the procedures of FIGS. 3A and 3B,the silicon nanowire polymers are formed as shown in FIG. 2D. That is,the silicon nanowire sheet or ribbon 25 is scooped up on the substrate28, and then irradiated with an ultraviolet ray 24 or the like as shownin FIG. 2E to cause one of crosslinking reaction and polymerizationreaction of the polymers 22, whereby a sheet or ribbon structuredmaterial is fixed.

Reasons for forming the silicon nanowire sheet or ribbon 25 on asubstrate such as glass or a flexible substrate such as a tape or asheet are as follows: handling of the sheet or ribbon is easy; a siliconnanowire structured material not depending on a substrate can beproduced; in a case where a silicon nanowire is formed directly on adevice, the device can be prevented from being damaged; the area of thesheet or ribbon can be efficiently used; and the like.

In addition, silicon nanowires preferably form a monolayer rather than amultilayer because electrical property thereof in the monolayer isstable. However, the present invention is not limited to Si wires of amonolayer or a multilayer. In addition, Si wires of the presentinvention may be doped with impurities. Thus, Si wires of the n-type orthe p-type can be applied to the present invention. Further, the Siwires may each have an oxide film including a natural oxide filmprovided on a surface of the Si wire.

The entire disclosure of U.S. Pat. Nos. 6,882,051, 6,036,774, 6,872,645and 6,835,613 is expressly incorporated herein by reference.

In addition, according to the present invention, there is provided aproduct including a plurality of silicon wires orientated in aparticular direction. In this case, the term “orientation” means thatthe longitudinal directions of the plurality of the Si wires arearranged in one direction.

The wires can be used for a channel of a field-effect transistor. Forexample, a group of the orientated Si wires are arranged between asource electrode and a drain electrode, and the orientated Si wires areused as a channel of a TFT. The term “group of Si wires” means aplurality of wires, that is, at least 2 wires.

In addition, the wires are described by taking the silicon wires as anexample. However, the present invention is not limited to the siliconwires, and can be applied to carbon nanotubes and nanowires including acompound semiconductor.

Examples

Hereinafter, specific examples of the present invention will bedescribed in detail. However, the present invention is not limited tothese examples.

Example 1

First, Au nanoparticles each having a diameter of about 20 nm wereallowed to be monodispersed on a silicon wafer, and Si was allowed togrow in SiH₄ gas through high-temperature CVD by using the monodispersedAu nanoparticles as catalysts to form silicon nanowires. Thethus-obtained silicon nanowires each had a diameter of about 50 nm and alength of about 5 μm.

Next, the silicon nanowires produced by the above-mentioned procedurewere put into a solvent together with a polymer as shown in FIG. 1A, andthe mixture was stirred for about 1 minutes by means of ultrasonic andthen left standing. In this case, polydibenzodisilazepine was used asthe polymer, and 1,2-dichloroethane was used as the solvent. In thesolvent after being stirred by means of ultrasonic and left standing,each of the silicon nanowires was covered with the polymer(polydibenzodisilazepine) and dispersed throughout the solvent as shownin FIG. 1B.

Next, the silicon nanowires each covered with the polymer were taken outfrom the solvent and put into a solvent b17 with affinity (in this case,an aqueous IPA solution), whereby the silicon nanowires each coveredwith the polymer floated on the surface of the solvent (FIG. 1C).Subsequently, an alternating magnetic field (1 MHz) of 500 oersteds wasapplied thereto, and the silicon nanowires each covered with the polymerwere then aligned as shown in FIGS. 1D and 1E. Next, the siliconnanowires were irradiated with an ultraviolet ray 14 for about 1 minutesto allow crosslinking reaction of the polymer to proceed, to thereby fixthe aligned silicon nanowires 15 (FIG. 1E).

Accordingly, a sheet in which silicon nanowires are aligned and arrangedwas formed by the present invention (FIG. 1E).

Example 2

The sheet (monolayer) formed of the silicon nanowires each of which wascovered with the polymer and which were floated and aligned on thesolvent with affinity, as described in Example 1 (FIG. 1C), was scoopedup and taken out therefrom on a polyethylene terephthalate (PET)substrate having a thickness of 100 μm (FIG. 2D). As shown in FIG. 3A,the sheet formed of the silicon nanowires each covered with the polymerwas scooped up in such a manner that the alignment direction of thesilicon nanowires was perpendicular to the direction of scooping-up ofthe PET substrate. In other words, the silicon nanowires 33 each coveredwith the polymer were arranged in a direction parallel to the appliedelectric field. Meanwhile, with respect to the LB film, the siliconnanowires were aligned by applying compression force in a directionperpendicular to the longitudinal directions of the silicon nanowires,and then the substrate was scooped up by moving the substrate in thesame direction as that of the compression.

The silicon nanowires aligned and arranged on the PET substrate asdescribed above was irradiated with an ultraviolet ray for 1 minute(FIG. 2E), whereby the polymer covering the silicon nanowires wassubjected to crosslinking reaction to fix the sheet.

Accordingly, a sheet 25 formed of the silicon nanowires aligned andarranged on the substrate 28 was formed by the present invention.

Example 3

The sheet formed of silicon nanowires each covered with the polymer andformed on PET (on the PET, an intermediate layer which sublimes byirradiation with a laser was formed (not shown)) as described in Example2 was turned upside-down and placed on electrodes (a source electrode65, a drain electrode 66, and a gate electrode 68) after formation ofthe electrodes as shown in FIG. 6A. A laser 63 was applied from above atop surface (substrate side) only to a part where the silicon nanowireswere intended to be arranged. The intermediate layer at the part wherethe laser was applied sublimed by heat, and only silicon nanowires 64each covered with the polymer in this area was transferred onto theelectrode side (FIG. 6B). In other words, the silicon nanowire sheet 61formed on the PET substrate 62 was successfully transferred onto thepart where the silicon nanowire sheet 61 was intended to be arranged (inFIG. 6A, reference numerals 67 and 69 denote an insulating layer and aTFT substrate, respectively).

Accordingly, the silicon nanowire sheet (without a substrate) of Example1 was successfully transferred onto a desired position on a device bythe present invention.

Namely, according to the present invention, there can be formed atransistor of parallel connection in which aligned silicon nanowires areformed at a desired position. In addition, since a plurality of thetransistor devices can be simultaneously produced on a glass substrate,the transistor devices can be used as switching transistors for pixelsof various displays, for example.

Example 4

For the sheet (LB film) of the silicon nanowires aligned in a monolayeron the solution as described in Example 1, using as a substrate a devicehaving electrodes formed thereon, a mask (not shown) in which an openingwas provided on a part where the silicon nanowires were intended to bearranged was prepared. The silicon nanowire sheet was applied and formedonto the desired position on the substrate by the LB method. In thiscase, the direction of alignment of the silicon nanowires was set to adirection from an electrode to another electrode (the same direction asshown in FIG. 6B). Next, unnecessary silicon nanowires formed on theelectrodes were removed by dry etching.

Accordingly, the silicon nanowire sheet (without a substrate) of Example1 was successfully arranged and formed at a desired position of adevice.

Comparative Example 1

Without covering the silicon nanowires formed in Examples 1 and 4 withthe polymer, a silicon nanowire film was formed on a substrate byutilizing only a flow of a solution (allowing the silicon nanowires tobe aligned approximately in the direction of the flow path), that is,the so-called LB method (FIG. 2D). As a result, as shown in FIG. 5,there were a number of the silicon nanowires 51 which overlapped with orcrossed over one another. In other words, in the present comparativeexample, there were formed devices having significant unevenness inelectrical characteristics among the devices, which depend on unevennessin channel-to-channel distances or unevenness in distances from the gateelectrode.

According to the present invention, the silicon nanowires can be alignedin a uniform direction. Thus, the silicon nanowires can be applied totransistors, whereby high-performance quantum apparatuses and advancedelectric apparatuses having stable electrical characteristics can beproduced.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

1. A method of producing a product including wires, comprising:dispersing the wires, which are covered with a polymer, in a solventhaving a polarity opposite to that of the polymer; arranging directionsof the wires so that each one of the wires is positioned end to end toone another; and fixing the wires whose directions are arranged end toend to one another.
 2. A method of producing a product including wiresaccording to claim 1, wherein the process of arranging directions of thewires is a process of applying at least one of an alternating electricfield and an alternating magnetic field to the wires.
 3. A method ofproducing a product including wires according to claim 1, wherein theprocess of fixing the wires is a process of applying thereto one of anultraviolet ray and an infrared ray or a process of applying heat.
 4. Amethod of producing a product including wires according to claim 1,wherein the polymer has insulating property.
 5. A method of producing aproduct including wires according to claim 1, wherein the wires arescooped up on a substrate from the solvent before the process of fixingthe wires.
 6. A method of producing a product including wires accordingto claim 1, wherein the wires are silicon wires.
 7. A method ofproducing a product including wires according to claim 1, wherein theproduct including wires as set forth in claim 1 is one of a monolayerwire sheet and a monolayer wire ribbon.
 8. A method of producing adevice comprising: preparing a wire sheet produced by the methodaccording to claim 1; preparing a mask having an opening; andtransferring part of the wire sheet onto a desired position using themask.
 9. A product comprising a plurality of silicon wires, wherein thesilicon wires are fixed with one another via a polymer so that each oneof the silicon wires is positioned end to end to one another.
 10. Aproduct according to claim 9, wherein the silicon wires each have on asurface thereof an oxide film.